1. Field of the Invention
The present invention relates generally to an error correction method and apparatus adapted for use in a teletext broadcasting system, in which digitally coded characters, patterns or the like are superposed on the television signal during vertical blanking periods, and more particularly to an error correction method and apparatus for restoring the coded information as completely as possible by correcting bit errors caused in a transmission path.
2. Description of the Prior Art
The realization of a practical teletext broadcasting system (teletext in Europe and North America) for displaying characters and graphs on the display of an ordinary home-use TV receiver set by using the present TV signal is progressing rapidly in various countries. In this system, the character and graphic information is converted to digital codes and multiplexed with the television signal during the vertical blanking period.
For teletext broadcasting, two methods are being considered. That is, a pattern method that transmits character and graphic information by dissolving the information into dot-patterns, and a code method of teletext broadcasting that transmits character and graphic information by changing the information into codes. The pattern method of teletext broadcasting was established officially as a standard method for teletext broadcasting in Japan in December 1982, after the Report by the Radio Technical Council of Japan was submitted to the Minister of Postal Services. On the other hand, with regard to the code method of teletext broadcasting, it is assumed that this method will maintain compatibility with the pattern method of teletext broadcasting so far developed, and that the code method will also permit various kinds of services in the future using coded signals.
The code method of teletext broadcasting has many merits, such as high transmission efficiency, high system versatility and high expansibility, etc., in comparison with the pattern method of teletext broadcasting. On the other hand, however, it has demerits such as the high price of ROMs to be used in the receiving set character generator. Furthermore if a bit error occurs in the transmission path, erroneous information will be displayed. However, according to recent progress made in LSI technologies and widespread use of word processors, etc., the ROM price will possibly be reduced by mass production.
In the Japanese character code broadcasting experimental system, which has already been announced, there has long been used the (8, 4) extended Hamming code and the (16, 11) extended Hamming code, in order to correct one bit error in a block including 8 bits or 16 bits. This system is adapted for use with an excellent transmission path in which less bit error is expected. This system, however, cannot sufficiently function to correct errors if there exist many impulsive noises, so that there exists a bit error during a period of several consecutive bits in one block, or if the transmission path characteristics are so poor that the bit error rate reaches as high as 10.sup.-2.
On the other hand, in the case of error correcting operations for character code broadcasting, the so-called "framing synchronized reproduction" for indicating the start of character codes in a packet is an important problem.
In the proposed system for reproducing the framing timing in a television receiver set, a predetermined bit pattern of a framing code transmitted immediately before the coded characters is detected for determining the framing timing, and thereafter the reception processing is made. With regard to this framing code, it is determined in advance that the distance between the framing code and the clock run in the packet (that is, the number of inconsistent bits between them) is equal to or more than three. Therefore, it is possible to correct only one bit error.
However, an actual television transmission path is not satisfactory as a digital data transmission line, so that it is possible that framing code errors are caused by impulsive noise, waveform distortions, ghost inteference, etc. Therefore, there has been a disadvantage in that, when receiving framing codes, various countermeasures must be taken by utilizing, for instance, characteristics of the framing code transmitted periodically and exactly.